How to animate the disappearance of a mesh
To get the cleanest result I suggest animated booleans. Since you have flat shading you wont get any shading artifacts, this is probably the textbook example for them. If you have some subsurf and displacement apply them or put the booleans after the displacement.

There will be 2 boolean objects.
- First one is static and will create the hole in the ground (highlighted). Put some edgeloops in the ground object around the trench so this boolean wont segment the whole thing.
- Second one can be just an animated cube and will intersect with first boolean object and the concrete wall inside trench. The bigger the intersection the more ground will be removed and more concrete wall will be shown.